Powered lift rotorcrafts such as helicopters and tiltrotors use lifting forces generated by wings or rotor blades that revolve around a mast. In a conventional rotorcraft, rotor blades are powered by one or more engines by way of a transmission, and the speed of the transmission input is reduced using one or more fixed ratio reduction stages such that the speed of the output powering the rotor is lower than the input speed by a fixed ratio. Optimization of rotorcraft performance, including noise, range, and efficiency, can be accomplished by varying rotor speed.
A typical method of reducing rotor speed in a rotorcraft is to reduce the input speed of a transmission provided by an engine, which directly reduces the rotor speed by a proportional amount. A general problem in performing this technique is that reducing the operating speed of the engine may result in a loss in engine efficiency or performance, degrading the net performance improvements possible by reducing the rotor RPM. This is mainly because the speed range of the engine that produces optimal power is more narrow and limited than that of the rotor system. In order to overcome these shortcomings, multi-ratio transmissions can be used to provide appropriate torque and speed to the rotor shaft by engaging gears of different ratios.
Friction clutches disengage the torque transfer path within a transmission and enable the engagement of an alternate gear system with a different reduction ratio. In such cases, torque is transmitted from the engine to the output shaft via frictional coupling between clutch plates.
While common for automobiles, the disadvantages of relying on friction to provide torque throughout speed transitions are; a lack of overrunning capability, increased debris generation, heat generation during engagement, installed weight (also referred to as power density), increased potential for drive train shock loads during engagement and disengagement, and potential for inadvertent disengagement while under load. Specifically, the need for additional components for overrunning capability necessary for aircraft autorotation makes the overall transmission assembly more complex and heavier.
From the foregoing, there is a need for a light-weight transmission shifting method and mechanism overcoming the above-described shortcomings of current fixed ratio and friction clutch based multi-ratio rotorcraft transmissions.